82077AA Datasheet, PDF (40/62 Pages) Intel Corporation – CHMOS SINGLE-CHIP FLOPPY DISK CONTROLLER

82077AA
The routine loops until RQM is 1 and DIO is 0 indi-
cating a ready status and FIFO direction is inward If
this condition is true the 82077AA is ready to accept
a command or parameter byte A timeout counter is
used to insure software response within a reason-
able amount of time in case of no response by the
82077AA As a note the programmer must be care-
ful how the maximum delay is chosen to avoid un-
necessary timeouts For example if a new command
is issued when the 82077AA is in the middle of a
polling routine the MSR will not indicate a ready
status for the next parameter byte until the polling
sequence completes the loop This could cause a
delay between the first and second bytes of up to
250 ms ( 250 Kbps) If polling is disabled this maxi-
mum delay is 175 ms There should also be enough
timeout margin to accommodate a shift of the soft-
ware to a higher speed system A timeout value that
results in satisfactory operation on a 16 MHz CPU
might fail when the software is moved to a system
with a 25 MHz CPU A recommended solution is to
derive the timeout counter from a system hardware
counter that is fixed in frequency from CPU clock to
CPU clock
For reading result bytes from the 82077AA a similar
routine is used Figure 8-2 illustrates the flowchart
for the routine ‘‘Get byte’’ The MSR is polled until
RQM is 1 and DIO is 1 which indicates a ready
status and outward FIFO direction At this point the
host can read a byte from the FIFO As in the
Send byte routine a timout counter should be in-
corporated in case of a disk controller lock-up condi-
tion For example if a disk was not inserted into the
disk drive at the time of a read operation the con-
troller would fail to receive the index pulse and lock-
up since the index pulses are required for termina-
tion of the execution phase
8 2 Initialization
Initializing the 82077AA involves setting up the ap-
propriate configuration after a reset Parameters set
by the SPECIFY command are undefined after a
system reset and will need to be reinitialized CON-
FIGURE command parameters default to a known
state after a system reset but will need to be reinitial-
ized if the system requirements are different from
the default settings The flowchart for the recom-
mended initialization sequence of the 82077AA is
shown in Figure 8-3
Figure 8-2 Get Byte Routine
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